Melting of Sedimentary Rock: A Key to Pegmatite Diversity in Maine

Last Updated: 21st Aug 2012

The Role of Migmatization in Maine Pegmatite Diversity

By Douglas Watts, Augusta, Maine.

[Updated to include some basic information on the Topsham, ME pegmatites.]

Outside of the Permian Topsham pegmatite district, which radiometric dating suggests is a temporally distinct igneous province from western Maine, there appears to be a positive correlation to mineralogical diversity in southwestern Maine pegmatites with regional migmatization (ie. complete melting) of the host country rock in which the pegmatites are emplaced. Two large migmatization 'fronts' have been tentatively identified in SW Maine. The first is associated with the Devonian Acadian orogeny (roughly 400-360 Ma); the second with the Carboniferous intrusion of the Sebago pluton (300-293 Ma). As a general rule, the most mineralogically diverse pegmatites in Maine are within and associated with these two zones of regional migmatization. Except for a unique outlier (Lord Hill), pegmatite mineralization becomes much simpler and beryl-dominated as one moves away from these migmatization zones both laterally and vertically.

Surprisingly, radiometric dating of Maine's mineralogically diverse pegmatites is almost non-existent; but in recent years good radiometric dates (U/Pb of zircon) have been obtained for the Sebago and Mooselookmegunticook plutons, the two most likely SW Maine pegmatite parents. Mooselook is dated at 377 Ma; Sebago at 300-293 Ma (Tomascak et al. 2005; 2008). Given this 77 Ma spread of emplacement, having U/Pb dates of the pegmatites would point to which parent each respective pegmatite came from. But that dating kind of misses the point.

The more fundamental point is that in recent years a much more detailed understanding of tectonic events in SW Maine during the period 400-290 Ma has arisen which suggests the 'old' structural model of a pluton jamming itself into 'cold' country rock in SW Maine and squirting out pegmatitic fluid hither and yon may not reflect reality. A more nuanced view, at least in SW Maine, is that its mineralogically diverse pegmatite bodies and fields may have evolved quite directly from the melting and chemical reorganization of in situ host meta-sedimentary rock, with the pluton itself contributing to this process primarily by localized and regional heating at great crustal depths. I favor this model if only because it offers a workable explanation for why none of the expansive Devonian plutons in eastern Maine (ie. Lucerne, Deblois, Bottle Lake) are associated with mineralogically diverse pegmatite fields. [1]

These two documented migmatization fronts are temporally displaced by ~75 million years but overprint upon the same geographic area in southwestern Maine. Absent radiometric dating of each and every SW Maine pegmatite, below I have used Phillip Morrill's 1958 western Maine pegmatite locality list, King and Foord's two volumes of the Mineralogy of Maine and my own field collecting to test the above hypothesis and see if it might have explanatory value.

Pegmatites in the Western Maine Migmatization Front

Pegmatites in the Sebago pluton migmatization zone include the Greenwood area pegmatites (Emmons, Tamminen, Waisenen, BB #7, Noyes Mt.), the Paris-Buckfield-Hebron group (Mt. Mica, Mt. Marie, Bennett, Hibbs, Mt. Rubellite), and the Mt. Apatite group (incl. Berry-Havey in East Poland). No complex pegmatites exist in Maine south of the Berry-Havey.

A second mineralogically and geographically distinct group is the Newry-Rumford pegmatites (Plumbago Mt., Black Mt., Bemis Stream, Goddards Ledge, Red Hill, Four Ponds Township, aka Harden-Keith prospect) which are assumed associated with the intrusion of the Mooselookmeguntic pluton to their north and west. Radiometric dating shows intrusion of the Mooselook pluton occurred at least 50-70 Ma prior to the intrusion of the Sebago pluton. It is assumed, but not proven, that the regional heating of the Sebago intrusion did not extend far enough north to affect the Newry-Rumford group, which means they crystallized well before the Mt. Mica/Buckfield, Greenwood and Mt. Apatite complex pegmatites, ie. they are temporally distinct. Of critical importance is that all of the Newry-Rumford pegmatites are at or near a similar elevation, ie. 1,000- 1,500 feet above msl, much higher in elevation than the presumed Sebago-related plutons to the southeast, which are all below 500 feet msl. No complex pegmatites are known in the Rumford area at or near ~500 feet msl (the basal elevation of Androscoggin River at Mexico); instead the bedrock at Rumford-Mexico at the Androscoggin River is migmatized meta-sedimentary rock with small lenses of simple granite and simple granite pegmatite.

The Peru-Hartford area pegmatites (Nate Perry Prospect, Lobikis, Hedgehog Hill, Ragged Jack) are somewhat anomalous and fairly simple in mineralogy, although the recently re-located Nate Perry prospect is unusually enriched in petalite and lithium-iron phosphates. Then there is a separate and distinct N-S band of simple, beryl-dominated pegmatites, usually quite small and exposed only as glacially scoured outcrops, which extend from the South Arm of Richardson Lake south through Andover and into Albany and Stoneham, the most significant being the Bumpus Quarry in Albany, famous for its 10-15 foot long beryl crystals. These are all at a fairly high elevation, ~1,000 feet. The Songo Pond quarry blue beryl deposit is anomalous. It is a more complex mixture of blue beryl, microcline, feldspar, but in some ways resembles the West Branch Ellis River vein beryl deposit to the north in Andover, esp. because it displays a noticeably pink microcline feldspar as matrix.

The Lord Hill, Coles Hill and Pingree Ledge pegmatites near the Maine-NH line are outliers enriched in fluorine (as topaz), triphylite, and phenacite. The Deer Hill-Colton Hill pegmatites and the Pleasant Mt. and Saltman amethyst occurrences due south of Lord Hill on the Maine-NH border geographically cluster as the only amethyst localities in Maine, with the Deer Hill glacial till deposits having fairly abundant 'Stoneham-Albany' type opaque pastel blue beryl. These occurrences appear distinct from miarolitic deposits at Mt. Moat and N. Conway in New Hampshire.

Of all the Maine pegmatite areas described above, one general pattern is they all share a fairly common abundance of beryl. The pegmatite areas of complex mineralogy have at least some beryl, and in the simpler pegmatites beryl is often the dominant mineral after quartz and microcline. As a general pattern, beryl in pegmatites with complex mineralogy tends to be massive and interstitial to matrix while in the smaller, simpler pegmatites it more commonly takes a typical, six-sided euhedral form. This cline is readily seen at Black Mountain in Rumford, where beryl in the lithium pegmatite is nearly always white, massive and almost indistinguishable from microcline, while in the much simpler pegmatites at the summit (and at nearby South Twin Mt. and Whitecap Mt.), it is euhedral to subeuhedral. Beryl is very poorly reported from Newry but mostly due to a lack of 'collector' quality specimens (in mass it was well confirmed and mined post WWII, especially at the Scotty Quarry).

Beryl is poorly reported from the western Mt. Apatite group (Keith, Wade, Pulsifer) and Berry-Havey, but is fairly abundant in the eastern group (Greenlaw, Maine Feldspar Quarry) where it is usually massive to subeuhedral and often in fairly large masses (up to 15 cm). Eastern group Mt. Apatite beryl is usually opaque to translucent in a range of very pale pastel greenish yellow and blue. Extremely pale purple to pink beryl scarce but present. At the eastern Mt. Apatite group, subeudral beryl is generally squat, ie. wider than it is long. At the MFQ beryl is generally absent from the lithiated zone and is more common in the quartz-microcline dominated areas, while at Greenlaw it is often alongside large black to bluish-black schorl xls.

As another general pattern, spodumene seems unusually concentrated in the Newry-Rumford zone, presumably associated with the Mooselookmeguntic pluton, rather than the Sebago pluton. The Mt. Plumbago, Newry pegmatite by far contains the largest amount of spodumene of any pegmatite in Maine, esp. at the Twin Tunnels excavation, where Dick Nevel reported 'picket fence' laths of greenish white spodumene nearly four feet in length. At Black Mt. quarry, much smaller white to pinkish purple spodumene laths (to several cm), are abundant in the rubellite, quartz and cleavelandite matrix. The very small but diverse Bemis Stream pegmatite is rich in 2-3 cm pearly white spodumene laths in a cleavelandite matrix with associated forest green elbaite. Spodumene is poorly reported in the Mt. Mica/Bennett pegmatites in Paris and Buckfield. At the Tamminen quarry in Greenwood, chalky masses of 'pinite' and montmorillonite are suggestive of highly altered spodumene. In the Mt. Apatite group, spodumene is poorly reported in the western group and at Berry-Havey. A few highly altered specimens are known from the Greenlaw Quarry (as yellowish 'damourite'). Of the entire Mt. Apatite group, only the Maine Feldspar Quarry pegmatite displays any considerable amount of spodumene, with several altered laths in quartz up to 12 inches in length and 3 inches in width. The least altered spodumene is a purplish gray to white, slightly translucent and chatoyant. Since most of the smaller, more durable pieces of spodumene are found loose in the MFQ dump piles, their direct mineral associations cannot be determined. The few large pieces of quartz matrix observed which contain spodumene show an affinity with masses of anhedral lithiophilite as well as montebrasite and minor elbaite. Lastly, the greenish muscovite replacement zone described at MFQ exhibits fairly large pseudomorphs after spodumene in a zone associated with very large schorl and complex euhedral smoky and milky quartz crystals. From observed specimens and pseudomorphs, the MFQ pegmatite appears to have had the largest spodumene crystals except for the truly outsized examples at Twin Tunnels, Plumbago Mt., Newry. The spodumene concentration at the MFQ and its association there with montebrasite and lithiophilite masses suggests the MFQ contained at least one substantial lithium and phosphate enriched 'core' zone bound in a massive quartz matrix. The comparative paucity of spodumene and lithia, iron and manganese phosphates in the Greenlaw quarry and its comparative abundance of colored elbaite, cleavelandite, lepidolite and 'fizzy' albite suggests the Greenlaw sill-like pegmatite was an outlying replacement body and the MFQ was more likely the 'core.'

Yet another pattern is indicated by the unusual enrichment of pollucite in the Mt. Mica, Mt. Marie, General Electric, Westinghouse and Bennett pegmatite complex in Paris and Buckfield as well as the Emmons-Tamminen group in Greenwood. Pollucite is basically unknown outside these two clusters of complex pegmatites (ie. from the Newry-Rumford group or Mt. Apatite group). Field relations indicate the Greenwood and Paris-Buckfield clusters are associated with the Sebago pluton migmatization front. Since the Mt. Apatite cluster is well within the Sebago migmatization zone, the absence of any known pollucite from there is anomalous.

Patterning of primary lithium and Mn/Fe phosphates are equivocal. Lithia Fe/Mn phosphates are abundant at Newry, present at Black Mt. quarry, present at the Black Mt. summit (as heterosite/purpurite in quartz and beryl), present at Goddards Ledge (ie. Ford Hill quarry) and abundant at Red Hill in Rumford, where triphylite is intimately associated with massive sphalerite. Triphylite is the presumed parent for eosphorite at Black Mt. The Bemis Stream pegmatite is absent of triphylite but has confirmed montebrasite. The Lords Hill and Coles Hill pegmatites at the ME/NH border are a clear outlier in triphylite abundance (where did they come from?). Lithiophilite is present at both Mt. Mica and Bennett in the Paris-Buckfield group. Rhodochrosite is uniquely enriched at the Bennett Quarry as outstanding pink euhedral masses and pocket crystals. The BB #7 pegmatite in North Norway is an odd outlier in that triphylite nodules are present as well as pockets of gem elbaite.

Small, altered rhodochrosite masses at the Berry-Havey quarry in East Poland (Mt. Apatite group) are the type locality for landesite. Lithiophilite is recorded as present at the Pulsifer quarry (western Mt. Apatite group). Discoveries by the author at the Maine Feldspar Quarry in 1995 indicate the Maine Feldspar Quarry pegmatite is highly enriched in lithiophilite along with some massive rhodochrosite and a very limited assemblage of secondary lithiophilite-derived minerals such as landesite, strunzite, etc. Nearly all of the observed lithiophilite at the MFQ is massive and unaltered. It is the author's surmise that the comparative abundance in primary Li/Mn/Fe phosphates at the MFQ quarry and their comparative scarcity at Pulsifer, Wade, Keith, Greenlaw and Berry-Havey is due to the former having a much greater volume and the latter being smaller and more sill-like in structure. As a further note, historic records suggest the Pulsifer, Wade, Keith, Greenlaw and Berry-Havey pegmatites were primarily explored for gem tourmaline pockets (and at Pulsifer, purple apatite) and, like at Mt. Mica, exploration ceased when miners hit the 'garnet line' at the base of the excavation, because they knew from experience it was unlikely that any tourmaline pockets would be found below the garnet line. In contrast, the MFQ pegmatite was primarily explored and mined for commercial ceramic-grade feldspar. From present field relations it is difficult to discern if miners at the MFQ blasted down and through the 'garnet line' there to extract feldspar, or if they even hit the garnet line. What does seem apparent is that the MFQ pegmatite was explored and blasted to a greater vertical depth than any of the other pegmatites in the Mt. Apatite group. What is intriguing, but unknown, is at what depth miners at MFQ hit the primary Li/Mn/Fe phosphate zone exhibited today by large masses of lithiophilite in 'bull' quartz. Dump relations suggest it was struck at a fairly significant depth.

Then there's apatite. Patterning of fluorapatite abundance in the SW Maine pegmatite field is quite interesting, but an important caveat needs to be mentioned. Virtually all pegmatitic fluorapatite records in SW Maine pegmatites are from avocational collection of visible euhedral crystals in miarolitic cavities. As such, pegmatite occurrences of fluorapatite outside of pockets and vugs would not be reported. This biases fluorapatite reports to those pegmatites with fairly abundant, accessible miarolitic cavities and against massive, non-miarolitic pegmatites, such as Black Mountain quarry. That said, collection records are robust enough to suggest a true fluorapatite scarcity in the Newry-Rumford group. Euhedral, 'showy' pale to deep purple apatite are well known from the Greenwood group, esp. the Harvard (Noyes Mt.) pegmatite (mostly purple) and the Tiger Bill pegmatites (mostly blue). Fluorapatite is present in the Mt. Mica/Bennett area, most notably as epitaxial crystals on well-formed schorl crystals at the 'Orchard Pit' in the Bennett pegmatite discovered by Ron Larrivee. The author has also observed a few 2-3 mm clear tabular fluorapatites at Mt. Mica from dump collecting in 1994. Since Mt. Mica fluorapatites were never large, blue, purple or 'showy,' it is likely few were ever preserved.

By far the highest concentration of fluorapatite in the SW Maine pegmatites is from the Mt. Rubellite in Hebron, Maine and the Mt. Apatite group in Auburn and East Poland. Mining by Jim Mann at Mt. Rubellite in the early 1990s revealed numerous epitaxial, miarolitic tabular lilac fluorapatites. The western Mt. Apatite group is the locus of all classic examples of purple apatite in Maine, particular Pitt P. Pulsifer's Purple Apatite Pegmatite Pit (say it five times fast). The eastern Mt. Apatite group is decidedly a lesser cousin in 'showy' purple apatite to the western group, but with a caveat. Apatite on the whole is highly abundant in the entire eastern complex, with a wide variety of colors, crystal habits and associations. Purple is rare, but pale lilac to greyish lilac (similar to Mt. Rubellite) is not uncommon at the very small Hatch Hill Farm prospect (privately owned). Medium to deep blue apatites (unf. very small, 2-4 mm) is fairly common in 'fizzy' albite near Greenlaw and is nearly identical in habit and albitic fabric at Berry-Havey pegmatite in East Poland. The 'Greenlaw Extension Quarry,' a very deep narrow slot to the SW of the Greenlaw quarry pit, features some 2-4 mm water-clear columnar fluorapatites in small vugs in massive cream yellow microcline.

But again, the Maine Feldspar Quarry pegmatite is by far the dominant pegmatite in terms of fluorapatite abundance, albeit mostly in massive and subeuhedral form. Here, in the 'giant dump,' fluorapatite takes several forms. One is intimately associated with large masses of dark brown lithiophilite. It is massive, deep olive green with lighter banding and conchoidal in fracture. The most common is as olive to medium green subeuhedral and roughly 'barrel' shaped crystals in cream-colored massive microcline from 1-4 cm. This is what is called 'manganoan' green apatite by collectors and is reported to be fluorescent. This phase commonly grades to bands of clear to purplish grey, especially on epitaxial surfaces, and is associated with 2-10 mm terminated 'swords' of columbite. The last phase, found in both Greenlaw and the MFQ, are 2-4 cm lozenge-shaped masses of opaque blue apatite, usually found near colored, non-pocket elbaite. If we assume that calcium is a key element driving apatite richness in the Mt. Apatite pegmatites, this to me suggests a substantial country rock contribution of calcium, presumably from calc-silicate pods, as compared to other complex pegmatites in the SW Maine field.

Last but not least is chrysoberyl. Poor, lonely, ignored chrysoberyl. Its distribution is suggestive but equivocal in the SW Maine pegmatite field. Two zones are apparent. First is the Witt Hill area at the Greenwood-Norway line where it is found in fairly simple pegmatites associated with almandine and spinel; also nearby at the very simple quartz-microcline-almandine Nubble Quarry in Greenwood, where the author found a medium green, opaque specimen nearly 2 inches across in 1993. The second is a 'zone' defined from Ragged Ass Jack Mountain in Hartford and southeast to several apocryphal (ie. lost) localities (McHugh and Conroy) in Mechanic Falls and Canton. At Ragged Ass Jack Mountain, chrysoberyl occurs as small thin, gemmy triangular yellow euhedral crystals in quartz in a narrow vertical pegmatite dike going straight up the sheer ~500 foot glacially plucked cliff alongside schorl and almandine. Extant written descriptions of the Canton and Mechanic Falls chrysoberyls describe them as fairly large and blocky (2-3 cm), euhedral and subeuhedral, opaque and olive green-grey in color. These descriptions are fairly similar to the occurrence at the Nubble Quarry. They do not resemble at all the Witt Hill and Ragged Jack Mountain occurrences. A non-chrysoberyl enthusiast would probably not even recognize them as the same mineral. What known relations exist suggest chrysoberyl is a rare and goofy outlier to otherwise simple tourmaline-almandine pegmatite dikes; its absence in more complex pegmatites is intriguing. But I'm not a chrysoberyl expert.

Taking the 3-D view

Known pegmatite exposures in the Rumford-Newry area offer a unique opportunity to examine pegmatite placement and development in SW Maine in a three-dimensional context due to the fairly high level of vertical relief in this area. Below I have cross-referenced USGS altitude data (in ft. msl) with approximate locations of known pegmatites in the Rumford-Newry areas from King (2000); moving from south to north starting with the Plumbago Mt., Newry pegmatites in Newry.

With the outlying exception of the Bemis Stream and Harden-Keith spodumene prospects, which are ~20-25 miles north of Rumford, all of the Rumford area pegmatites follow a 15 mile N-S strike of 1,500-2,500 foot mountains and hills bounded to the south by the NW-SE flowing Bear River valley (Rt. 26) and to the north by North Twin Mtn. at Mine Notch at Rt. 120 in Andover. The Androscoggin River valley forms an eastern control with a basal elevation of 500 ft. msl at Mexico and ~650 ft. in fairly flat intervale above Rumford upper falls. The large, flat glacially scoured and filled bowl-shaped valley of the Ellis River forms the western control of this mountain mass.

In this distinctive pegmatite zone, altitude data shows a consistent patterning of somewhat simpler pegmatites at the 1,200 foot level (Elliot ppt., Goddards Ledge, Red Hill; Carver ppt.); the Plumbago Mt. and Black Mt. complex pegmatites at 1,500 feet; and the simple quartz muscovite beryl pegmatites at the 2,000-2,300 ft. level, represented by beryl-microcline-quartz outcrops at the peaks of Whitecap Mt., Black Mt. and S. Twin Mt. What is truly odd is the absence of any known pegmatite exposures in the area from the basal elevation of the Androscoggin River at Rumford and Mexico (500-650 ft. msl) up to the 1200 ft. level. [3]

These relations appear to indicate a very horizontal, sill-like intrusion pattern. Most notable is the similarity in quartz-beryl-muscovite-microcline exposures at and near the summits of Whitecap, Black Mt., and S. Twin Mt. and the readily evident sill-like pegmatite exposure which forms the remarkably flat-topped and 'park-like' summit of Whitecap Mountain. The similarity in pegmatite mineral and fabric and elevation at all three summits is interpreted to represent a fairly long (5 mile) horizontal sill intrusion which capped each mountain. It is interesting to note that the two largest and most complex pegmatite bodies of the group, at Black Mt. and Plumbago Mt., share a common altitude of 1,500-1,600 feet. The group at the 1,200 ft. elevation are much less complex and generally smaller. The Elliot ppt., on the southern 'back' shoulder of Mt. Dimmock, is very unusual for having a fairly large zone of white cleavelandite which is strangely barren of accessory minerals. Red Hill and Goddards Ledge share an affinity of Mn/Fe phosphates (triphylite), most notably at Red Hill, and a fairly high sulphide content. Goddards Ledge is unusual for a fairly substantial deposit of rose quartz, usually iron-stained [presumably due to weathering of pyrite and other sulphides.] Due to long-term access issues (ie. posting), these two pegmatites are severely understudied and few specimens exist from them. Noticeably absent at both is spodumene, which is a major part of the Newry and Black Mt. complex pegmatites, although all four have triphylite in common.

The Bemis and Four Ponds (Harden-Keith Spodumene Ppt.) pegmatites southeast of Mooselookmeguntic Lake are true outliers, being the northernmost lithia pegmatites in Maine and separated by a N-S gap of nearly 30 miles from the Rumford-Newry group described above. The author has only visited and studied the Bemis Stream prospect, but reported minerals indicate the Harden-Keith pegmatite is very similar. The Bemis Stream pegmatite is quite similar in mineralogy and fabric to the spodumene phase of the Black Mt. quarry pegmatite with the exception that rubellite is absent at Bemis Stream and only forest green elbaite is observed [at Black Mt. quarry spodumene laths similar to those at Bemis Stream is found almost exclusively with cinnamon pink rubellite]. Cleavelandite is similar in size and fabric at both sites in the spodumene association, although the cleavelandite at Black Mt. has a pronounced sky-bluish coloration, while the Bemis Stream cleavelandite is pure white. The close proximity of the Bemis Stream and Four Ponds spodumene pegmatites to the Mooselook pluton, and their total isolation from all other pegmatites suggests an intimate connection to the Mooselook pluton. Of note, which may be purely coincidental, is that the altitude of the Bemis Stream pegmatite (~1600 ft.) very closely matches that of Black Mt. and Newry pegmatites, and all three are spodumene rich. The Four Ponds spodumene pegmatite is a vertical outlier, at ~2,300-2400 ft. msl.

The paucity of pegmatites outside the known localities in this area (and the large gap from the Rumford pegs. to Bemis Stream), might be explained solely by lack of identification. However, early 1900s to WWII records suggest the area was quite well 'scoured' for significant pegmatite bodies during the period in which commercial feldspar and mica production was profitable. While some exposures certainly might have been missed, the distribution pattern based on known occurrences seems to be the result of true emplacement patterns rather than mere 'gaps on the map.'

A second clue comes from close examination of eroded cobble and boulders and bedrock ledge outcrops which comprise the bed of the Swift River, which extends from Mexico north for ~30 miles to its watershed terminus near the Bemis and Four Ponds pegmatite areas. Truly pegmatitic cobbles are virtually absent in the entire river course, suggesting a true scarcity of pegmatite bodies along its strike.

A third clue arises from intensive field study and mapping of outcrops in the Swift River by Solar and Brown (2006) which defines a basal migmatization zone from Mexico to Roxbury and a steadily decreasing cline in metamorphic grade as one moves up the river valley. Because of the very steep slope of the valley, this horizontal cline in metamorphic grade upriver also corresponds to a vertical cline as one moves 'up' in the bedrock. At Mexico, the basal level of the Swift is 500 feet msl. At Roxbury, it is 800 feet msl. At Coos Canyon in Byron it is 900 feet. At the junction of the E. Branch Swift River it is 1000 feet. At the junction of Berdeen Stream with the Swift River at Houghton the elevation is 1315 feet, an increase of 800 vertical feet from Mexico. Based on Solar and Brown's mapping, the decreasing cline in metamorphic grade up-valley in the Swift most likely has both a vertical and horizontal component. By horizontal mapping, the migmatization zone in Mexico and Roxbury dissipates below Coos Canyon in Byron, but this also represents a 400 foot vertical increase in basal elevation [the rocks in the Swift River in Mexico are 400 feet deeper than those at Coos Canyon; the rocks at Coos Canyon are 400 feet deeper than those at Houghton]. Given these data, unteasing the verrtical vs. horizontal component of the cline in metamorphic trend (from almost fully melted rock in Mexico to staurolite-grade, well-bedded meta-sedimentary rock at Houghton) is a bit problematic.

Given these data, one might craft a simplistic vertical model wherein the migmatization of bedrock observed at the mouth of the Swift River at Mexico (elev. 500 feet msl), represents where meta-sedimentary rock was deep enough to be melted and to express melted silica-rich fluid moving upward. As they moved upward in the Rumford area, these melts created broad, horizontal silicic and pegmatitic sills at various levels above the migmatization zone which spread laterally in surrounding country rock in a 'layer cake' fashion, thus resulting in a sequential, elevation controlled series of sill-shaped pegmatite deposits at the 1200 ft., 1600 ft. and 2000 ft. levels on N-S strike from Plumbago Mountain to S. Twin Mt. What this model does not help explain is the curious fact that the most mineralogically complex pegmatites (Black Mt. quarry, Newry, Bemis Stream), all at 1600 ft. msl, are vertically sandwiched between a more simple group below them (at 1200 ft.) and above them by a much simpler beryl-muscovite group at 2000-2300 ft. (Whitecap Mt. summit, Black Mt. summit, S. Twin Mt. summit.)

Elevation analysis of the Andover-South Arm blue beryl exposures, due 15 miles west from Rumford-Newry are not too helpful. Blue beryl are exposed in bedrock in the W. Branch Ellis River at the Andover-Andover North Surplus town line at the mouth of the N-flowing Frye Brook at 1,000 feet msl. The Sawyer Notch/Moody Mtn. blue beryl locality due north is at ~1250 ft. msl. The latter locality is poorly reported (only from Morrill 1958) and it is uncertain if this deposit is from talus coming from much higher elevations in the area. The W. Branch Ellis River locality is useful since the beryl are in bedrock in the stream and the elevation of the stream is fixed at 1000 ft. msl.

While the great difference in elevation at these Andover sites (~1000-1200 ft.) from the location of the beryl-rich sill in Rumford (+2000 ft.) suggests the two are not genetically related, field specimens and fabric from the W. Branch Ellis River show a number of similarities with specimens from the Black Mt. and Whitecap summit exposures. The W. Branch Ellis River beryl are in bedrock in the bed of the river, allowing for close examination of the host rock. Generally, the beryl occurs in small lenses of simple pegmatitic quartz and microcline and muscovite in the streambed or in waterworn boulders with the same fabric. Beryl color ranges from a pale to deep blue-aqua or medium sea-green and is highly euhedral. Observed crystals are 2-4 cm dia. Some of the associated microcline is noticeably pinkish and resembles microcline at the Songo Pond blue beryl pegmatite south of Bethel. Getting any real control over potential relations with these Andover blue beryl deposits and the Rumford-Newry group 15 miles east is difficult due to the lack of availability of specimens reliably identified to source. Morrill (1958) is the only reliable source of beryl localities in the Andover area; few if any known specimens still exist (ie. well-labelled to locality).

The Topsham-Bowdoin Pegmatites

The Topsham-Bowdoin pegmatites appear to be temporally, mineralogically and tectonically distinct from all others in SW Maine and require separate treatment. First, they are provisionally dated by U/Pb analysis of monazites at the Standpipe Hill pegmatite to be emplaced at ~270 Ma, which is much younger than the 300-293 Ma emplacement of the Sebago pluton to the west, making the Topsham field the youngest of complex pegmatites in Maine. Second, their high concentration and diversity of U/Th and REE minerals makes them anomalous among complex Maine pegmatites. Last, and perhaps most important, the Topsham-Bowdoinham pegmatites outcrop along a very narrow NE-SW zone extending from Brunswick to the south and north to the Richmond-Bowdoin line, which never crosses the Flying Point Fault. The pegmatite line pinches out at the north end of Bowdoin (Dingley Road) and to the south at Brunswick (LaChance Quarry).

The FPF marks the boundary of the Silurian marine turbidites of the Central Maine Sequence (CMS) and unrelated and older volcanogenic gneisses of the Falmouth-Brunswick sequence (FBS) of mid-coast Maine and Casco Bay. Despite decades of detailed mapping by Hussey, West, Cubley and others, the true relationship of these two large NE-SW swaths of metasedimentary rock is still unclear in the Topsham-Bowdoin area. In the Portland area rocks across the Flying Point Fault show a distinct change in metamorphic grade suggesting significant (< 2 km) Mesozoic uplift at the fault contact. But in the Bowdoinham area, the transition is much more cryptic, although clearly observable at the field level by rock type. The Hornbeam Hill foliated gneiss, dated to 393 Ma, appears to form a stitching boundary between the rocks of the CMS and the FBS (Nehumkeag Pond formation) in the Bowdoin-Richmond area. This dating requires that tectonic movement (up and down or lateral) in this area stopped with the main Acadian deformation in the early and mid Devonian. But radiometric dating of the Topsham pegmatites show they were emplaced 120 Ma after Devonian Acadian deformation and 20 Ma after the emplacement of the Carboniferous Sebago pluton approx. 20 miles to the west and across the CMS-FBS boundary. Most importantly, the Topsham-Bowdoin pegmatites exhibit a curious and stubborn allegiance to only outcropping within the Falmouth-Brunswick sequence parallel to the strike of the CMS-FBS boundary. This is completely weird since the 393 Ma dating of the Hornbeam Hill gneiss suggests the CMS-FBS 'join' had been stitched and static for over 100 million years before emplacement of the Topsham-Bowdoin pegmatite. The null hypothesis is that the Topsham-Bowdoin pegmatites should have been randomly emplaced on both sides of the CMS-FBS strike, ie. without regard to this very old and static boundary. But they do the opposite: they hew consistently and only to the FBS side of the join. To make things even weirder, West and Cubley (2006) mapped small schorl and almandine pegmatite pods in the Bowdoin USGS quadrangle, but only on the CMS side of the unconformity, never on the FBS side. These pegmatites are undated but are mineralogically distinct from the Topsham-Bowdoin field just a few miles across strike to the east.

But what do the minerals tell us? The minerals and fabric of the Topsham pegmatite are unique in Maine. They are fairly simple in mineralogy but often extremely outsized, ie. euhedral microcline crystals to 1 m in dia. and cores containing cubic yards of pure, transparent to smoky quartz. After quartz, microcline, biotite and muscovite, beryl and almandine are the dominant but scarce accessories. Lithium is nearly non-existent and the pegmatites are curiously depleted in schorl. Ferrocolumbite is scarce, but when found is in masses larger than any found in Maine (xls and masses up to several pounds). U/Th and REE minerals are unusually abundant, esp. rare species (ie. Ishikawaite, Xenotime-Y). Uraninite is present as 1-3 cm glossy, unaltered complex euhedral crystal groups, but also as highly altered masses of pitchblende with numerous secondary U/Th alteration minerals. Monazite-Ce is unusually common, up to xls of 1 cm. While U/Th and REE minerals are not completely absent from other SW Maine pegmatites to the west, their presence and diversity is very subdued as compared to Topsham.

A few clines are apparent in the Topsham field. One cline is that unaltered, large uraninite crystals appear familial to the simple biotite-rich part of the field. As muscovite becomes the dominant mica species, euhedral uraninite becomes absent or much smaller and highly altered to pitchblende. A second cline is that magnetite bipyramids are peculiar to the Standpipe Hill-Mt. Ararat area, especially in aplite, but are rare to non-existent to the north. Ishikawaite appears uniquely common to the Standpipe Hill pegmatite but is rare to non-existent elsewhere. In general, the macroscopic presence of REE and U/Th minerals decreases from south to north, with the Coombs Quarry in Bowdoin, an otherwise large and diverse pegmatite, showing no REE and U/Th minerals. The Fisher Quarry, which is the most complex and most lithiated of the pegmatites is nearly barren of U/Th and REE minerals, except for very minor allanite-Ce. Just to the north on strike, at the Alice Staples pegmatite, altered U/Th minerals, monazite, radioactive zircon and ferrocolumbite are common at joins of almandine, nearly black smoky quartz and muscovite, suggesting extreme pegmatite fractionation at a small (500 m) scale, including fractionation of U/Th minerals. The Topsham pegmatites are also unique in SW Maine for the presence of molybdenite and bismuthinite. Molybdenite is known from simple quartz pegmatite dikes at the head of tide falls of the Androscoggin River at Brunswick, where it was first collected and reported by Parker Cleaveland, and also in the 'Square Pit' ~5 miles to the north. In both localities, the molybdenite is in 0.5-1.5 cm subeuhredal flaky masses in either rusty quartz (Androscoggin River falls) or quartz-microcline pegmatite (Square Pit). These two occurrences seem to confirm a genetic association with the 'Brunswick' granite and the Topsham pegmatite field, which is further supported by radiometric dating reported by Carl Francis and Paul Tomascak (1995). Bismuthinite, a sulphide virtually unknown from other SW Maine pegmatites, was reported in fair abundance at the Standpipe Hill pegmatite by Morrill (1958) and recollected there by the author in the 1994. Bismuthinite was also collected at the Alice Staples Quarry 1/4 mile north of the Fisher Quarry in 1995 by the author as one acorn-sized lump. The presence of bismuthinite at both sites further confirms a genetic association with the Standpipe Hill pegmatites and those ~5 miles to the north on strike in the Fisher Quarry area.

The Alice Staples Quarry (the name is acc. to Gene Bearss) is oddly enriched in sphalerite as interstitial masses corroded into gold muscovite plates along with interstitial masses of gahnite. At the Coombs Quarry pegmatites ~5 miles north in Bowdoin, sphalerite is also found as 1 cm glossy subeuhedral masses in greenish olive microcline along with massive pyrite. By my own naive viewpoint, when I see massive sulphides in a SW Maine complex pegmatite I think of anatectic melting of a euxenic 'black shale' metasedimentary host as the potential source of sulphur. That said, the anomalous presence of molybdenum and bismuth sulphides in the Topsham pegmatites appears to further argue for their wholly distinct evolution from other SW Maine pegmatites. Another theory, which I will just throw out for the hell of it, is whether the unusually high U/Th enrichment of the Topsham pegmatites might be attributed to the melting of euxenic black shales, which are often U-enriched (Van Baalen 2006). The odd presence of fuchsite in schist at Wolf's Neck State Park in Freeport, fairly on strike to the S of the Topsham pegmatites, is intriguing since chromium is another element known to concentrate in euxenic black shales. Lastly there is apatite. Apatite is virtually absent from the Topsham pegmatites except at the Fisher Quarry, where it appears in two different phases. The first is a 'water clear' phase of 3-5 mm complex tabular crystals in tiny vugs in albite; the second is as tiny quartz shaped transparent blue crystals in the cleavelandite matrix of the Fisher Quarry topaz pocket. The Fisher quarry topaz pocket was obviously enriched in fluorine, hence topaz, and this Fl-enrichment seems to be a logical explanation for the anomalous presence of fluorapatite in and around it. The tantalite concentration in the pocket zone (as emerald green microlite and grey stibiotantalite) is very weird. So even at a very small map scale, the Topsham pegmatites defy simple classification. I'll deal with hyalite opal next.

Conclusion

The most basic, child-like question about a pluton is where the rock went which occupied the physical space the pluton occupies. It can either go up, go to the side or become incorporated into the pluton itself or all three. But it has to go somewhere. The enormous size of many exposed plutons in Maine (ie. Katahdin, Sebago, Deblois, Lucerne, Mooselook) requires displacement of the rock which was where the plutons were before they intruded. This rock had to 'go' somewhere else. Where did it go? Contact metamorphic aureoles around plutons in Maine are well documented. Shatter zones, well displayed on Mt. Desert Island in east coastal Maine, document how a physically eruptive upwelling magma 'bubble' can crack and break up the cold host and partially incorporate pieces of the rock as xenoliths into the melt. Field relations at the contact zone of mineralogically diverse SW Maine pegmatites have never been well studied, in part because the major interest of study was the economic value of the pegmatites themselves. The contact at the Bell Pit and Twin Tunnels pegmatites at Plumbago Mt., Newry is particularly fascinating because the pegmatites intruded into a pre-Acadian gabbro pluton, creating an aureole of large (3-8 cm) brownish black sprays of dravite crystals in the gabbro matrix. At Black Mt. quarry, the roof pendant of biotite schist is riddled with 3 x 10 mm schorl crystals growing parallel to the schist foliation. [2] King and Foord (1994; 2000) report euhedral vesuvianite crystals at the contact aureole of the Tamminen pegmatite in Greenwood, and Leavitt (same volume) reports vesuvianite crystals in the contact aureole of the Sturtevant pegmatite in Minot. But in general, all of the mineralogically diverse pegmatites in SW Maine display quite short and abrupt contact aureoles and easily read zoning within the pegmatite from the contact to their core. Rather than a diffuse 'mixing' zone with the country rock, the physical bounds of the pegmatites are always easily discerned. This fabric does not, in my opinion, discredit migmatization of country rock as the primary source of these pegmatites but is more a statement that at some point a demarcation is physically required, and there definitely was 'injection' of melt into otherwise cooler country rock, but the melt was most likely from the adjoining rock itself.
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Notes:

[1] You could boil this whole paper down to saying that mineralogically diverse pegmatites in Maine appear to require regional metamorphic facies at low to high sillimanite or K-feldspar levels, but that takes all the fun out of it.

Comments

Doug:
This is interesting. Not having dug in Maine much I'm not that familiar with the geography, thus without maps it is difficult for me to follow your discussion. I dont know if there is a way for you to include one or two maps in these articles. But in any case, in the 1980s, David London published a map of the pegmatites of the Middletown district here in Connecticut that showed how the complexity of the pegmatites varies systematically from a core area of simple, basically coarse-grained granites (I), through pegs with only graphic texture further out (II), then simple zoned pegs with some accessories constituting most of the district (III), and finally limited intermittent areas of the most complex and elementally diverse pegmatites (IV) such as Walden, Strickland, Gillette, western White Rocks & Riverside, Swanson. To get from an area of type I pegs to an area of type IV, you have to pass through areas of types II and III pegs. In his recent book Pegmatites, London notes (using different terminology) that the type IV represent the most evolved, and therefore furthest from the source rock, pegmatites. They arent associated with migmatization here. This finding suggests that the geographic variation of pegmatite complexity represents a cross-section of the vertical variation that is now tectonically domed. I mention all this because you might want to make a similar map(s) for the Maine pegmatite district(s) and see what it might tell you. I can email you a copy of the London map, modified by me based on experience with pegmatites he didnt include, but which do not violate the systematic variation.

I was at Twin Tunnels and Bell Pit a couple of years ago and loved finding tons of spodumene! I had not collected any in decades since the closing of Strickland Quarry (hisssssss) and at first didnt even recognize the "strange, woody" mineral I was holding! I'll be there again Aug. 18 searching for columbites, but probably taking home more spodumene...

BTW, I recall collecting at Keith Q. several years ago and finding several buckets worth of yellow beryl all over the blasted rock, many in cabinet-specimen sized hunks. The PMC guide said that heliodor is not on the list of minerals from there and that it was all microcline. Well, there was plenty of yellowish-tan microcline laying about too, but as some of my specimens were distinctly hexagonal, and having collected in pegmatites since the 1970s (and been trained as a geologist in the 80s), I know I can tell the two minerals apart, especially since the beryl lacked any perthitic texture! The one with the columbite in it was particularly glassy, too. After some "discussion", we ended up agreeing to disagree, I took a few home and gave the rest to other collectors there. So not only might there be collector bias in the under-reporting of beryl from there, there may also be an "institutionalized" bias that reinforces the former. I'll post some photos of the "non-existent" heliodor soon.

The age of the Sebago Pluton has been of some interest to me and the partial melting of metasediments in Maine. You might like to refer to the following. I've mentioned these ideas during talks at the Maine Mineral Symposium as well as in Collector's Guide to Granite Pegmatites. The melting of metasediments certainly does seem to be a likely origin of Maine pegmatites and is a revival of earlier ideas by Montgomery (1950) and others.